Recently, a projection display device (LED projector) has been proposed using a light emitting diode (LED) as a light emitting element for a light source.
An LED projector requires an image display with high brightness, and for this purpose it is essential that etendue determined by the light emitting area and the angle of divergence of the light source not be increased. That is, to utilize light from a light source as projection light, it is necessary for the product of the light emitting area and the angle of divergence of the light source to be smaller than the product of the area of the display element and the acceptance angle (solid angle) determined by the F number of the projection lens. In an optical system where light is projected to a light modulation element, it is accordingly necessary to improve use efficiency of light from a light source by enhancing the directionality of outgoing light.
On the other hand, in an LED projector that displays an image using a light modulation element such as a liquid crystal panel, it is necessary to project a specifically polarized light onto the liquid crystal panel because of polarization dependency of the liquid crystal panel. Therefore, it is also necessary for the above optical system to convert a randomly polarized light from an LED light source into a specifically polarized light and to output it.
Patent Literature 1, as such an optical system, discloses the planar lighting system that converts randomly polarized light from an LED light source into specifically polarized light and outputs it.
This planar lighting system has a polarization separation film provided on the exit side of a light guide plate, which polarization separation film is formed by sandwiching a metal thin film between transparent media having a low refractive index. In this polarization separation film, surface plasmon is excited on a surface of the metal thin film via an evanescent light that is produced when light is totally reflected off an interface between the light guide plate, i.e. the transparent medium having a high refractive index, and the thin film having a low refractive index. Then, light that can excite surface plasmon is only light that has an electric field component parallel to the propagation direction of the surface plasmon and an incidence angle based on dispersion of the permittivity of the medium on both sides of the interface in which the surface plasmon propagates. Further, the exit side and the entrance side have the same configuration (the transparent medium having a high refractive index and the thin film having a low refractive index), which develops an inverse process to the excitation process of surface plasmon on the exit side, thereby allowing a linearly polarized light that excites surface plasmon to be transmitted and other polarization components to be reflected. In such a manner, by transmitting only the linearly polarized light incident on a metal layer at a predetermined incidence angle, which satisfies the excitation condition of surface plasmon, an output of polarized light with an improved directionality is realized.